The development of purely chemically driven visible lasers had been an unattained goal for at least the past twenty years. Visible chemical lasers are difficult to implement because the chemical processes which directly establish requisite population inversions leading to lasing are rare. Until now, the only chemical processes known and used to establish population inversions have lead to lasing primarily in the infrared spectral region, the furthest extension toward the visible being to 1.3.mu.. The infrared chemical laser systems cannot be used to produce visible lasing, and an entirely new set of processes had to be developed to create the purely chemically driven visible laser of the present invention. As a result, the process of this invention is wholly not described in any prior art and, as such, there is no known relevant prior art.
Classical laser operation in general requires a population inversion in which the upper energy level associated with the lasing transition is more populated than is the lower energy level on which the transition terminates. Laser oscillation can be established in an optical cavity which allows photons to be reflected back and forth and interact with each other so as to build up the intensity of the radiation. A select group of lasers, including primarily N.sub.2 which operates in a pulsed mode under amplified spontaneous emission, does not require such an optical cavity as the photon amplification is so large that sufficient intensity is produced without the necessity of mirrors. A further technique involves stimulated Raman pumping in which an intense laser beam is converted into a beam of another frequency by coherent Raman stimulation in a two or more step scattering process. Each of these lasers operates on electrical energy.
Various types of infrared chemical lasers are known of which there are two main types. The first type involves the mixing of an oxidizer and a fuel gas to produce a continuous output. The mixture is activated by an electrical discharge or by thermal decomposition induced using arc heaters or combustors. The activated mixture produces a reaction initiating species, the reaction sequence eventually leading to a population inversion and lasing involving one of the constituents of the mixture. The second type uses premixed fuels and oxidizers which are activated by flash photolysis with an electron beam or a pulse discharge. However, once the mixture is ignited, it may present flameout and detonation problems and may be difficult to extinguish. Hydrogen halides and carbon monoxide are the two main types of molecules used as lasing species in these chemical lasers.
A typical chemical laser is disclosed in U.S. Pat. No. 4,553,243. This laser operates by expanding the reactant gas mixture continuously through a supersonic nozzle and applying a pulsed electrical discharge to initiate the chemical reaction resulting in the production of the lasing species. The frequency of the electrical pulses can be adjusted so as to regulate the frequency of the laser. The gas mixture is introduced on the fly, usually at pressures from a few Torrs to multi-atmospheres. This laser does not operate as a purely chemically driven system as it requires an electrical discharge to initiate the chemical reaction. More importantly, this laser only operates in the infrared region and cannot produce visible lasing.
A chemical oxygen-iodine laser using iodine chloride as a reactant gas is disclosed in U.S. Pat. No. 4,563,062. In this device, iodine chloride is vaporized and entrained in argon gas. This gas mixture is directed into a laser cavity where it is mixed with singlet oxygen. Upon mixing with singlet oxygen, the iodine chloride dissociates into atomic iodine and atomic chlorine. Subsequently, the atomic iodine is excited to a lasing state through collisions with the singlet oxygen. These lasers are typically operated with a laser cavity pressure in the range of 1-3 Torr. Using the oxygen-iodine system one produces a laser which operates at 1.3.mu., in the infrared region. Such systems have not been developed to produce visible lasers.
A typical semiconductor laser, based on the gallium arsenide semiconductor, is disclosed in U.S. Pat. No. 4,446,557. This type of laser requires that an external electric field to be applied using an electrode located on the semiconductor layers. When the external electric field is applied, photons are created which resonate among the semiconductor energy levels so as to produce lasing action. The laser produced by a semiconductor system is at a much longer wavelength than the laser of the present invention and operates among much lower lying energy levels.
Self-pulsing, semiconductor lasers which have a pulsed, rather than a continuous wave output beam are also disclosed in U.S. Pat. No. 4,446,557. However, it is difficult to reliably reproduce devices having a very high pulse repetition rate with extremely good temporal stability. Certain geometries for the semiconductor cavity length directly related to the principal noise resonance wave length have been suggested to alleviate this problem. Self-pulsed semiconductor lasers have the same limitations as the typical semiconductor lasers mentioned above.
Metals having sufficient vapor pressures at relatively low temperatures can be made to lase. To create the vapor pressure necessary for lasing action, metals have been heated in electric or gas fired furnaces to approximately 1675.degree.-1875.degree. K. The large amounts of metal vapor required to make such a laser practical require considerable electric power for heating, thus making the resultant laser very bulky and not readily susceptible to mobilization. The use of a gas fired furnace, which is more mobile than an electric furnace, lessens this problem to some degree but the system is still bulky. The use of either an external oven or discharge heating to produce the high temperatures of between 1675.degree. and 1875.degree. K. makes it difficult to construct the fast discharge circuitry needed for excitation of other self-terminating neutral metal laser transitions. Using metal halides helps to reduce the temperature requirements to some degree. The invention which we report for certain applications requires oven systems operating at temperatures in excess of 1600K., however, for example, the source of silcon or germanium atoms required to produce metastable storage states which make operative several of the systems of the present invention may be obtained from gaseous silane or germane oxidation reactions as noted in the following Detailed Description of the Invention. A number of the metal atoms required as energy recipients and subsequent atomic lasants can be obtained from sources operating at temperatures considerably less than 1600K. In addition, the technology needed for operation at the higher temperatures required to operate the particular sources considered here is readily available.
Another commonly used method for creating metal vapor lasers is to sputter the metal atoms from a cathode of the desired material. Control of the sputtering process has been achieved by entraining the sputtered metal in a gas stream so as to create a metal vapor beam. This metal vapor beam is directed into an optical cavity where a separate electrical discharge system excites the metal vapor. Generally, the metal vapor beam is passed through a ring shaped electrode in order to minimize the electrical discharge necessary to excite the lasing constituents in the beam. Metal vapor lasers are not premised on chemical processes such as those reactions used in the present invention. First, there is no chemical reaction. Second, when based on the metal halides, they generally employ dissociation processes caused by an external laser. Third, they are largely operative in the infrared region with only a few examples operative at shorter wavelengths.
Metal halide pulsed lasers capable of simultaneously providing a plurality of output beams oscillating at discrete wavelengths in the visible and near infrared portions of the spectrum are disclosed in U.S. Pat. No. 4,607,371. Such a plurality of output beams is obtained through the dissociative excitation of a number of vaporized metal halides composed of the Group II B metals. Excitation is achieved either by photo-dissociation or by dissociation through collisions with energetic electrons produced in a transverse discharge or by an electron beam generator. The power of such lasers can be enhanced by using isotopically pure metal halide salts rather than their naturally abundant counterparts. As such, this laser relies on a dissociation process caused by an external laser and not a chemical reaction.
Chemically driven visible lasers offer attractive alternatives to their infrared counterparts; however, the development of a chemically pumped system lasing in the visible region, while occupying the interest of researchers for almost two decades, represents a difficult problem whose solution has met with little success until the present invention. This invention focuses on the development, extension, and detailed quantification of visible chemical laser systems and the demonstration and quantification of laser amplification and oscillation across the visible and ultraviolet regions employing purely or primarily chemically pumped systems. The development of such devices necessitates innovative approaches to the generation of electro-magnetic radiation. To implement these approaches, we take advantage of the unique features associated with certain high cross-section, highly selective exothermic reactions and several new insights gained in the study of ultra fast energy transfer processes in small high temperature molecules.
Efforts toward the goal of a visible chemical laser oscillator are to be encouraged for not only is there reason to pursue these systems for their potential high gain, but also, given similar power levels, a device based on an electronic transition holds advantage over those based on infrared transitions in that the size of the device may be smaller, the power consumption efficiency larger, and the optics considerably simplified relative to the infrared. Devices once constructed and optimized can play an important role in a diversity of field based operations. Finally, chemical lasers once developed are inherently more efficient than systems based exclusively on electrical power input.